Dip the chicken fillets in the nonfat yogurt until well coated, cover and let them marinate in the refrigerator for 3 to 4 hours.

Heat the char-grill. Brush the yogurt from the chicken fillets and discard it. Dry the chicken pieces on paper towels, then grill them, until very brown and no pink juices run out when pierced with a fork. Allow 15 to 20 minutes, turning them once.

Meanwhile, fry the onion in the oil until soft and starting to brown. Add the garlic and ground cumin and continue cooking over medium heat for 2 to 3 minutes, stirring constantly.

Purée the onion mixture with the creamy yogurt in a food processor or blender. Season with salt and pepper. Stir in the fresh coriander, then heat the mixture gently in a small saucepan. Do not boil the sauce, or it will separate.

Taste the sauce for seasoning and keep warm. Pile the chicken onto a serving dish and spoon over the sauce. Serve hot or at room temperature.